Mild winter increases risk of mosquito-borne diseases

Farmers in the Southeast are bracing for increased insect threats to their crops, created by an unseasonably warm, and in some areas dry, winter. In addition to crop threats, the unusual weather pattern brings with it some human risks and greater livestock risks that are heightened for rural areas.

Eastern Equine Encephalomyelitis (EEE) and West Nile Virus (WNV) are two mosquito-vectored diseases that are now endemic to rural areas of the Southeast. Though the greater risk for either disease is to livestock, there is significant risk to humans from both diseases.

David Marshall, a veterinarian with the North Carolina Department of Agriculture says the best way to protect both animals and humans is to plan now — in the early spring — to reduce breeding grounds for mosquitoes and to vaccinate animals that may be exposed to either of the diseases.

“Horse owners should talk with their veterinarian to determine best times to start the vaccination program, both of which require two vaccinations, 30 days apart for horses with no history of the disease,” Marshall says. Neither vaccination fully protects the animal until after the second shot, so it is a good idea to start as early in the mosquito season as possible, Marshall explains.

Typically, animals and humans are at greatest risk from EEE and WNV from July to October. Florida is the only Southeastern state reporting cases of either, or both, of these diseases year-round.

West Nile Virus was first detected in humans in the United States in 1999. Now, it is found in all 48 contiguous states and in neighboring areas in Canada and Mexico. Since its first detection in the U.S. 15,000 cases and 500 deaths have been recorded.

By comparison, the first human case of EEE in the U.S. was diagnosed in 1938. This disease is found primarily east of the Mississippi River, with southern states being in the highest risk category.

Though animal numbers aren’t always a reliable indicator of the number of humans affected by these diseases, there is a common connection, probably related to the number of mosquitoes infected with the two viruses.

In North Carolina, the number of EEE and WNV cases has dropped dramatically, since 2003. So, have the number of human cases. In 2004-2005 only six equine WNV and 20 EEE cases were reported in horses. Combined, in 2003, nearly 250 cases of the two diseases were reported in the state’s horse population.

Better awareness of the diseases and better vaccination programs have been a primary reason for the dramatic drop in equine and human cases across the Southeast. However, periods of cold weather in 2003-2004 and 2005 also contributed to the reduced number of mosquitoes available to vector the disease.

Typically, these viruses begin in wild birds, primarily crows, hawks and to a lesser degree bluejays. In each epidemic over the past 10 years, the start of the disease is linked to a swampy area, with adjacent woodlands that provide ideal habitat for wild birds.

Infected birds are bitten by mosquitoes, which in turn carry the disease to other birds and mammals and to humans. In a typical mosquito population, much less than one percent are infected by the viruses.

In horses, WNV symptoms include loss of appetite and depression, fever, weakness or paralysis of hind limbs, convulsions, and impaired vision. In humans the vast majority of those infected have no symptoms, those who do, usually have flu-like symptoms.

EEE symptoms in horses include aimless wandering, inability to swallow, impaired vision, convulsions, and in a high percentage of cases, death. There is no evidence that either disease can be transmitted to humans or to other animals in any way other than by mosquitoes.

Eliminating standing water that serves as mosquito breeding grounds is the best way to combat these diseases. In the peak periods for infection — July to October — staying indoors at prime mosquito feeding times (at dawn and at dusk) and using insect repellent are good ways to reduce human risks.